The present invention relates to a method of operating electrolytic cells utilizing a source of electric current having varying electrical load levels. More in particular, the present invention relates to the operation of a plurality of electrolytic diaphragm cells for the electrolysis of aqueous alkali metal chloride solutions by a method suited to the use of interruptable or off-peak power.
Electrolytic cells have been used extensively for many years for the production of chlorine, chlorates, perchlorates, caustic, hydrogen and other chemicals. Over the years, such cells have been developed to a degree whereby high operating efficiencies have been obtained. One of the more recent developments in electrolytic cells has been in maintaining high operating efficiencies while drastically increasing the current capacities at which the individual cells operate. The increased production capacities of the individual cells operating at high current capacities is advantageous, providing higher production rates for any given cell room floor space, thus, reducing capital and operating expenses.
The present method of operating electrolytic diaphragm cells may be utilized in various processes. Chlor-alkali diaphragm cells are presently of primary commercial importance, and, therefore, the present invention will be described in terms of such cells. However, it will be understood that the following description is not to be interpreted as limiting the usefulness of the present method to chlor-alkali diaphragm cells.
Typically, an electrolytic cell installation consists of a plurality of cells electrically connected serially together in groups called circuits. Usually, circuits consist of from about 10 to about 100 cells. In the case of diaphragm cells, each cell has an anode and a cathode separated by a diaphragm of fluid-permeable, corrosion-resistant material. Suitable diaphragm materials are asbestos, resins or mixtures thereof. In the case of a chlor-alkali cell, an aqueous brine (sodium chloride) is fed into the anolyte compartment, and, upon the application of an electrolyzing, or decomposing current to the electrodes, gaseous chlorine is produced at the anode and sodium hydroxide and gaseous hydrogen are produced at the cathode. The sodium hydroxide is dissolved in the cell liquor leaving the cell from the catholyte compartment. Chlor-alkali cells have been developed and are now in commercial use which operate at current levels of 150,000 to 200,000 amperes.
The major expense of operating chlor-alkali cells is electrical current. Typically, sources of electrical current, utilities, have a relatively fixed capacity to supply power, while the demand usually has large daily and seasonal variations. The differences between capacity and periods of low demand is termed interruptable or "off-peak" power and is available at a lesser rate if the purchaser agrees to accept load decreases when the demand otherwise increases. The operation of electrolytic cell installations utilizing interruptable power is highly desirable from an economic standpoint; however, such use of interruptable power has not heretofore proved successful in installations utilizing diaphragm cells because the necessary and frequent variations in the load have caused permanent tightening of the cell diaphragms to the extent that liquid flow through the diaphragms is severely curtailed. The loss of diaphragm porosity requires the entire cell circuit to be subsequently operated on a reduced electrical load to prevent severe operating difficulties. The result of such diaphragm tightening is that the total original production capacity of the installation is reduced until the overly tight diaphragms are replaced. The present process provides a means of maximizing the use of available interruptable power while minimizing the possibilities of such operating difficulties.